The present invention is generally directed to an apparatus for interrupting a circuit at a predetermined time. The present invention is more particularly directed to an in-line electronic timer which provides improved safety and energy conservation by interrupting a circuit after a predetermined time and by drawing no current and dissipating no power after interrupting the circuit.
Electronic timer circuits have often been used for interrupting a circuit formed between a source of alternating current (an AC source) and a load. Such timer circuits generally include a timer, a control circuit and a switch. When the switch is actuated, the circuit is completed between the AC source and the load, allowing current to flow to the load. The timer is powered by the AC source and begins timing when the circuit is completed. When the timer has measured a predetermined time, the timer generates a timeout indication. In response to the timeout indication, the timer circuit interrupts the circuit, decoupling the load from the AC source.
One particular use for such a timer is in conjunction with a wall switch which controls room lighting. Actuation of the wall switch provides alternating current to the lighting and starts the timer. When the timer times out, the circuit is interrupted, turning off the lights. Such electronic timers are important for conserving energy by automatically disconnecting the load, such as lighting, from the AC source.
However, the energy-conserving capability of such prior art in-line timer circuits has been limited due to the power consumed by the timer circuit itself. Even when the circuit is interrupted and little or no current flows from the AC source to the load, prior art timer circuits consume power. Some prior art timer circuits employ a relay to interrupt the circuit. When the timer times out, the timer energizes a relay coil to actuate a relay switch and interrupt the circuit. Even after the timer times out, however, the timer circuit consumes power in the form of the holding current which continuously energizes the relay coil to hold the relay switch open.
Another prior art timer uses a triac, a bidirectional thyristor, which converts from a high impedance "blocking" state to a low impedance "on" state in response to an applied control current. When the timer times out, the control circuit removes the control current and the triac changes to its blocking state, interrupting the circuit. Such a timer uses two power supplies, one which is operating when the triac is conducting and one which is operating when the triac is blocking. Thus, the timer still draws current from the AC source, even when the circuit to the load is interrupted.
Such prior art in-line timer circuits which draw current even when the circuit to the load is interrupted also pose a safety hazard.
Such timer circuits may allow a small current to flow in the circuit after the timer times out. This small current may be insufficient to activate the load, for example, to cause a lighting system to provide illumination. However, such timer circuits may be used in conjunction with AC sources which generate potential differences as high as hundreds of volts. At such high voltages, even a very small current passing through the timer circuit to the load may be dangerous to a person working with equipment that forms the load. Such a person may assume the timer circuit has decoupled the load from the AC source since the load is not apparently active. Such a person may be injured by the small currents conducted by the timer circuit which are insufficient to activate the load.
The present invention overcomes these limitations and provides other advantages over the prior art. The present invention provides an electronic timer which automatically switches an electrical load off after a programmed time. The present invention consumes no power after interrupting the circuit and blocks all current flowing to the load. The present invention may be adapted to operate in conjunction with both standard (i.e., two-way) and three-way switching circuits.